The simplest definition of friction is: The force F, needed to tow an object, creating a certain pressure N, against a flat horizontal surface.

Mathematically the friction coefficient: µ(mu) = F / N

When friction is measured at airports according to ICAOprocedures in Annex 14 and associated documents the friction measured shall be the MAXIMUM FRICTION. This friction is measured by having a certain slip.

Slip is defined: Slip = V ”free rolling wheel” - V ”test wheel” /V ”free rolling wheel” Slip,S, is normally expressed in percent. 100 % slip thus means that the braked wheel is skidding. Maximumµ (mu) is measured between 10 and20 % slip. More about slip is found in Section 3.1.3 below. Friction characteristics of a runway is very dependent on the Structure of the runway surface. One differentiate between Macro-texture and Micro-texture. A good macrostructure should have a mean depth of about 1 mm. The micro-texture is dependent on the aggregate used when the surface is being built. E.g. limestone will give very low micro-structure and a runway having such aggregate may become very slippery, especially when wet.

The speed has great effect on the friction of the runway when wet. If both macro-texture and micro-texture are poor the effect on friction of a runway when wet will be extremely dependent on speed. Naturally, if only one of the texture kinds is poor the effect of speed on friction will be less

Author Information - Gunnar Antvik

Since the late forties Mr. Gunnar Antvik have been involved in friction measurements at airports. He began as a Manager of Performance Engineering of Scandinavian Airlines Systems, SAS. Therefore, airport friction characteristics were of concern to him. He then developed still used methods of determining friction characteristics at airports. These methods are all included in ICAO annex 14.
Mr. Gunnar Antvik was the spokesman in matters related to friction movements areas on airports for SAS, and in many cases also for IATA and for the Swedish Civil Aviation Administration as well. In the early fifties ICAO commissioned a work group that was lead by Mr. Gunnar Antvik. As a result the first guidelines for friction testing were included in Annex 14, 1952. In 1965 Mr. Antvik joined the Swedish Civil Aviation Administration, mainly to be able to represent Sweden at ICAO meetings. He was also appointed to be a member of the Runway Surface Condition Study Group, RSCSG. This group prepared material for ICAO meetings on friction measuring at airports around the world. Over the years Mr. Gunnar Antvik has accumulated considerable knowledge of challenges related to runway friction measuring.

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